Removal issue for praefect['virtual_storages'] deprecation syntax

In GitLab 13.9 gitlab-org/omnibus-gitlab!4988 (merged) we changed the syntax for specifying nodes in praefect['virtual_storages'] and in 13.12 we added a deprecation warning: gitlab-org/omnibus-gitlab!5240 (merged) that notes that the old method will have support removed in GitLab 15

In 15.0 we can remove support for the old method